1. Field of the Invention
The present invention relates to a reciprocating, spark-ignited internal combustion engine operating on gaseous hydrogen.
2. Disclosure Information
Hydrogen has long been recognized as an engine fuel offering interesting possibilities. On the one hand, hydrogen combustion produces very few hydrocarbons (HC), carbon monoxide (CO), and carbon dioxide (CO2) because there is no carbon in the fuel. Rather, carbonaceous exhaust constituents arise from small amounts of lubricating oil participating in the combustion event. Hydrogen is thus a desirable fuel from an exhaust emissions standpoint. Because of its combustion characteristics, in particular its extremely lean flammability limit, it is possible to operate a hydrogen fuel engine at extremely fuel lean air/fuel ratios. For the purposes of this specification, discussion will be made of equivalence ratio, Ø which in common automotive parlance means the ratio of the stoichiometric air/fuel ratio to the actual air/fuel ratio. Accordingly, values of Ø equivalence ratio less than one correspond to lean air/fuel ratios, and equivalence ratios greater than one correspond to rich air/fuel ratios. With a system according to present invention, purging of an NOx trap will at rich air/fuel ratios (i.e. e. Ø) regardless of engine speed and load.
A problem with the use of hydrogen as a motor fuel arises due to the high flame speed of hydrogen. Although the stoichiometric air/fuel ratio for hydrogen is about 34.2:1, in the absence of recirculated exhaust gas (EGR), most engines cannot be run at at Ø i.e.,stoichiometric air/fuel ratio) because of problems with auto ignition. Simply stated, auto ignition is the tendency for the mixture within the combustion chamber to self-ignite prior to the arrival of the flame front propagating through the mixture as a result of the spark ignition. Because of concerns related to auto ignition, engines fueled with hydrogen have been required to run at extreme fuel lean conditions (air/fuel ratio of about 65:1) in order to prevent auto ignition and to provide stable combustion. However, if high levels of EGR are used, an engine may be able to operate at at Ø albeit with a concomitant loss of fuel economy. Accordingly, it is desirable to operate at at Ø in order to maximize fuel economy. Another problem associated with known hydrogen fuel engines arises from the need to maintain compression ratio at relatively lower values in order to avoid the previously described auto ignition. In one engine which has been the subject of published research, the engine was operated with hydrogen at a compression ratio of about 10:1. The present inventors, have determined, however, that it is possible to operate an engine on hydrogen at 14:1-15:1 compression ratio, provided the engine is controlled according to the present specification and claims.
A hydrogen fueled reciprocating spark ignition engine includes a fuel system for providing gaseous hydrogen to the engine""s cylinders, and a lean NOx trap coupled to the engine for treating the engine""s exhaust. An EGR system provides recirculated exhaust gas to the engine""s intake, and a controller operates the fuel system and the EGR system during periodic purging of the lean NOx trap such that the engine is operated at or near a stoichiometric air/fuel ratio, with the mass of EGR approximating the mass of air and fuel. The fuel system and the EGR system may be operated by the controller to purge the lean NOx trap on either a timed basis or by integrating the output of a NOx sensor mounted within an exhaust system of the vehicle or engine.
If a timed basis is used to schedule purge operation, the EGR system and fuel system may be operated to purge the lean NOx trap for about 3-5%, for example, of the engine""s operating time.
According to another aspect of the present invention, a hydrogen fueled reciprocating spark ignition engine may have a fuel system and an EGR system controlled during operation at or near maximum load so as to achieve an air/fuel ratio and EGR rate which are comparable to the air/fuel and EGR rate utilized during purging of a lean NOx trap mounted in the engine""s exhaust system.
According to another aspect of the present invention, the present hydrogen fueled reciprocating spark ignition engine may further comprise a three way catalyst mounted either upstream or downstream of the lean NOx trap. In the event that additional reductant is needed during purging because an engine having a system and being operated according to the present invention cannot tolerate sufficient hydrogen to produce needed reductant in the exhaust, the engine may further comprise an auxiliary fuel system for providing hydrogen to the engine""s exhaust system upstream of the lean NOx trap.
An engine according to the present invention may be operated at an equivalence ratio of about 0.15 to about 0.65, except when the lean NOx trap is being purged, in which case the equivalence ratio will be greater than 1.0 with heavy EGR to avoid pre-ignition. In this case, the EGR mass flow rate will be about 40%-80% of the total mass flow entering the engine""s combustion chambers.
According to another aspect of the present invention, a method for operating a reciprocating internal combustion engine equipped with a lean NOx trap and burning gaseous hydrogen, includes the steps of operating the engine at an equivalence ratio of about 0.15 to 0.65, with the precise equivalence ratio being a function of engine speed and load. When purging a lean NOx trap, the equivalence ratio will be set at about 1.1. The present method may further include the step of operating the engine with the mass of EGR being approximately equal to the mass of air and fuel when the lean NOx trap is being purged. The present method may further comprise the step of operating the engine with the mass of EGR being approximately equal to the mass of air and fuel when the lean NOx trap is being purged and when the engine is operating at or near maximum load.
It is an advantage of a system and method according to present invention that hydrogen fuel may be used so as to exploit hydrogen""s best characteristics by operating extremely lean to achieve maximum fuel economy except when regeneration of the lean NOx trap is required, wherein the engine may be operated at a richer-than-stoichiometric air/fuel ratio with heavy EGR, which causes a reducing atmosphere had to be present at the front face of a lean NOx trap, so as to allow excellent regeneration of the lean NOx trap.
It is another advantage of the present invention that an engine operating according to the present system and method may utilize higher compression ratios than those typically found with hydrogen fueled engines. In an engine operated according to the present invention, the hydrogen and air are mixed with a high percentage of recirculated exhaust gas, in the range of 40-80%, by weight. These exhaust gases help to prevent the autoignition of the hydrogen and air. In effect, the exhaust gases are a diluent which does not participate in the combustion reactions to produce an exotherm; instead, the exhaust gases absorb combustion energy and cause the peak temperature of the burned gas mixture to be lower. Additionally, the exhaust gases contain water, which interferes with the chain reactions leading to autoignition.
It is yet another advantage of the present invention that an engine operating according to the current system and method maybe operated without the need for aftertreatment other than a NOx trap.
Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification.